1. Technical Field
The invention relates to a load brake comprising a load shaft, a flange-like pressure plate fixedly mounted onto the load shaft, a cogwheel arranged on the load shaft with tension means which produce an axial motion and force in the cogwheel as the wheel rotates with respect to the load shaft, a ratchet wheel mounted between the pressure plate and the cogwheel, a pawl for preventing the rotation of the ratchet wheel at least in the release direction of the load shaft, friction surfaces provided between the pressure plate and the ratchet wheel, and the ratchet wheel and the cogwheel, whereupon rotation that tends towards the release direction of the load shaft and that is faster than the speed of rotation of the cogwheel brings the pressure plate and the cogwheel closer together in the axial direction and presses them against the ratchet wheel, which results in the braking of the rotational movement of the load shaft, whereas rotation that occurs in the release direction of the load shaft, that is directed at the cogwheel, that exceeds the braking torque and that is faster than the speed of rotation of the load shaft removes the pressure plate and the cogwheel further apart in the axial direction and therefore unlocks the braking connection of the pressure plate, the ratchet wheel and the cogwheel.
2. Background Art
This type of load brake is used especially in a hoist gear unit of a hoist, where the purpose is to ensure that the movement of the load is stopped and the load remains suspended even when the primary brake of the hoist, which usually means a brake provided on the shaft of the hoist motor, is damaged and cannot therefore stop or hold the load.
The load brake does not brake during the hoisting, but during the descent of the load there is in the brake a torque which equals at least the load torque and against which the motor and the load work. This work is converted into heat, which is harmful for the operation of the gear since the properties of the lubricating oil in the gear deteriorate at high temperatures.
Consequently, during the descent of the load there must be in the load brake a torque that is at least equal to the load, so that the load can be controlled, if necessary, even when the primary brake or the primary step of the gear is damaged. In conventional load brake constructions, a sufficient brake torque has been ensured by sizing the elements of the load brake such that the torque produced by the load in the brake greatly exceeds the torque corresponding to the load. As a result, more energy is converted into heat in the load brake than just the potential energy of the load. This reduces the degree of utilization of the application (e.g. a hoist) since the lubricating oil of the gear reaches its highest allowed working temperature with a lower degree of utilization than if the oil were only heated by the potential energy of the load. Further, the motor is required to compensate for the difference between the torque of the load brake and the load torque, which means that the motor does more work than if the torque of the load brake were closer to the load torque. The motor therefore warms up too much unnecessarily.
The load brake is in principle a construction that is formed of a "screw" and a "nut" and a "non-rotating element", which is provided between the former two but which is generally allowed to rotate during the hoisting in the direction of hoisting so that the brake does not drag. The component that allows the rotation in the hoisting direction of the load and that prevents the rotation in the opposite lowering direction of the load, i.e. in the direction in which the load shaft is released, is a one-way switch. The "non-rotating element" and the one-way switch may be referred to herein as a ratchet mechanism which consists of a ratchet wheel and a pawl, as mentioned at the beginning.
Friction surfaces are attached to the head of the "screw" and to one side of the "nut", these surfaces resting on the surfaces of the ratchet wheel. The load torque tries to turn the "screw" into the "nut", which produces a compression force on the friction surfaces. The torque of the load brake is therefore a function of the geometry and friction coefficients of the friction surfaces and the screw system. If the "nut" rotates when the "screw" is tightened, the axial force cannot increase and the brake is not provided with a sufficient torque. On the other hand, the aforementioned geometry and friction coefficients also affect the degree of tightness of the load brake during the normal operation of the hoist. At worst the brake may be jammed so that the torque of the hoist motor cannot unlock it. In such a case, the load remains suspended in the air and it must be removed from the hook or the load brake must be opened manually. The friction surfaces may also be attached to the ratchet wheel, in which case the heat that is generated due to the friction is transferred to the rotating parts and from there to the oil.
The combination of a "screw" and a "nut" can also be replaced with a "camnut" construction, which is a special kind of screw-and-nut combination consisting of two elements that rest one against the other and that have corresponding cam surfaces. If a load brake has such a construction, the brake may start vibrating when a small load is lowered. The reason for this is that the hoist motor unlocks the load brake so rapidly that a small load does not have time to rapidly move one half of the "camnut" out of the way, but the cams collide. This collision in turn accelerates the load so that the brake is relocked and the motor must unlock it once again. Some load brake manufacturers have tried to eliminate this drawback by providing an axial force between the friction surfaces, this force being independent of the load and producing in the brake an initial torque that is constant. This does prevent the brake from vibrating but it increases the heat generation of the brake and it does not ensure that the load torque tightens the brake.